The present invention relates to improvements in the manufacture of reclosable flexible containers, and more particularly relates to a method and apparatus for sealing zipper closures along the side seams of the containers during the manufacture thereof.
Reclosable flexible containers are well-known in the art. Such containers normally comprise a bag-like structure made from a folded web of thermoplastic film. Both single and multiple ply laminated films are commonly used. Zipper closures, comprising interlocking male and female zipper closure elements, may be formed and secured to the underlying film web in a number of ways. The zipper closure elements can be formed by extruding each zipper closure element with the film and then interlocking the elements to form the zipper closure. The closure elements may also be separately extruded, interlocked, and then fused or otherwise attached to the film. Or, the closure elements may be separately extruded, fused or otherwise attached to the film, and then interlocked to form the zipper closure. When the zipper closure elements and film are separately extruded, the zipper closures or zipper closure elements and film are typically brought together and fused while still warm.
A problem has existed in the art in providing good quality seals in the areas on the sides of the containers adjacent to the zippers and to the ends of the zippers themselves. The provision of zipper closures on the film web leads to difficulties in the manufacture of the containers since the plies of thermoplastic material then include portions of nonuniform thicknesses.
One attempt at addressing this problem is that before flexible containers are formed from the web of thermoplastic film, the film, with zipper closures or separate zipper closure elements fused thereon, is subjected to a spot-sealing step. Referred to variously as stomping, welding, and pre-sealing, the step of spot-sealing comprises the application of heat and/or pressure to the zipper closures or separate zipper closure elements in an area in which the film is ultimately to be sealed and severed into individual bags to flatten out additional thickness which is presented by the zipper closures. This area of spot-sealing is ordinarily wider than necessary to accommodate variations in position of the film when it is sealed and severed into individual flexible containers.
Spot-sealing has the effect of reducing the thickness of the zipper closures vis-a-vis the thermoplastic film, which reduction enhances the manufacture of leak-proof zipper end seals. The step of spot-sealing may occur either as an early step in the manufacturing of flexible containers, for example as when such containers are made on vertical packaging machines, or as a step in preparing a supply of thermoplastic film with zipper closures or separate zipper closure elements spot-sealed thereon for later use on container manufacturing or packaging equipment, such as rotary drum machines or horizontal packaging equipment.
Typically, in the manufacture of flexible containers, the web of thermoplastic film is folded into a U-shaped configuration which defines a bottom edge and opposing side-walls or plies. A bag-like structure is created when the folded film is then sealed and severed into individual bags. So formed, the zipper closures are located near but spaced below the top edge of the flexible container.
Variations in this basic manufacturing process arise where products are packaged and hermetically sealed in the flexible containers at the point of manufacture. By way of example, on vertical packaging machines, a step-wise process is involved wherein, first, the web of thermoplastic film and zipper closure elements are extruded and fused; second, the film is advanced to a station for spot-sealing; third, the film is folded and the zipper closure elements brought together; next, one side seal is formed; then, the product inserted or the bag filled; the second side seal is completed to enclose the product; and, finally, the first and second side seals are severed to separate the packaged products. On horizontal packaging machines, for example, the film may more typically be supplied with spot-sealed zipper closures in place, the film wrapped around a product, and the side seals completed, all in a continuous, rather than step-wise, operation.
Simplification of flexible container manufacturing processes and packaging processes has long been the object of attention in the industry. Such simplification desirably reduces the cost and time required to produce reclosable flexible containers or to package products in such containers. Of further concern has been the occurrence of inadequate zipper end seals in the side seals of flexible containers produced on conventional machines. Despite spot-sealing techniques, leakage of container contents and migration of bacteria into the containers has been found to occur through inadequate zipper end seals.
Inadequate zipper end seals have arisen as a result of the crushing pressure of opposing seal jaws used to form the side seals of flexible containers. Such crushing pressure has been found to distort zipper closures, resulting in poor zipper closure properties near the flexible container side seals. Attempts to obtain good hermetic and fluid-tight zipper end seals have not been satisfactory. Reduction of the crushing pressures imposed by opposing seal jaws on the zipper closures may result in areas between the raised zipper closures and the film at the zipper end seals remaining unsealed. Compensatory increases in sealing temperatures to cause plastic to flow into these areas has resulted in the delamination of individual plies of composite thermoplastic materials used in many flexible container applications.
Accordingly, further improvements in manufacturing zipper end seals have been a continuing object of attention in the art, and further improvements are needed to attain the desired leak-proof zipper end seals.